In an overhead travelling carriage system, articles are conveyed by allowing an overhead travelling carriage to run using a running rail installed near a ceiling of a clean room or the like. In a large-scale system, the route is divided into conveying routes in a process (intra-bay routes) and conveying routes between processes (inter-bay routes). A plurality of intra-bay routes are connected to one inter-bay route. In a normal system, a stocker is placed between each intra-bay route and the inter-bay route. Thus, a conveying operation is divided into a part from a processing device to the corresponding stocker, a part from the stocker through the inter-bay route to a stocker located at the boundary between the inter-bay route and the intra-bay route to a destination, and a part from this stocker to a load port for the processing device at the destination. This requires a large number of stockers, and temporary storage of articles in the stocker is repeated before the travelling carriage reaches the destination. Consequently, the conveyance requires a long time. In connection with this, the Japanese Patent No. 3508130 proposes that a processing device facing one of the intra-bay routes conveys an article directly to a processing device facing the other intra-bay route. However, the conveyance is restricted by the possibility that the load port at the destination is occupied. For example, if the load port at the destination is being used and the conveyance of the article is assigned to an available overhead travelling carriage, it is difficult to predict whether or not the load port will be free before the overhead travelling carriage arrives. If the load port is unavailable for a long time, the conveyance of the article is delayed.